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Old Aug 08, 2012, 03:58 PM
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Joined Jul 2011
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104" PBY Catalina. Need some motor/prop suggestions.

Hey guys.

So I'm building a 104" Catalina with retracts. It will take off of land and water. It's time to put together the power system. I need someone a lot smarter than me to recommend the some good motors/props to use on this thing.

The specs are:

-104" wingspan
-approx flying weight may be about 18 lbs.... I know... heavy. I have some lightening to do. ....I might be able to get it down to about 14 lbs.
-I haven't figured out wing surface area but I'll get that calculated and post it ASAP.
-I am limited to 12" props due to clearance with the body. And of course I will be doing three blade props. I will have them counter rotating even though the real catalinas aren't I just don't want any funky flight characteristics.

I'm going for a sport, light aerobatic type of flying. I want power to pull the plane out of bad situations and to pull it off the water.... obviously I won't be doing much maneuvers with it.

...OK... so fire away. I need some guru's to tell me what motor/prop/batter/ESC I can use to get enough thrust out of that size of prop. .....p.s.... did I mention this is my first real electric plane? ...until now I've been a gas guy.
I've read everything I can about electric setups I just don't trust myself to do the math correctly.... plus I'm limited to a prop size and it's a twin so I'm not certain how that affects the setup

Thanks in adavance

-Chris
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Old Aug 08, 2012, 09:01 PM
I think I'm inverted. Maybe.
acetech09's Avatar
United States, CA, Pacifica
Joined Apr 2012
1,505 Posts
Welcome to the E-world.

There's a rule of thumb about watts per pound. X weight doing X flying stile should have a motor around X watts. It's a rough estimate, but pretty handy. For a scale Catalina, you'd want anywhere around 110 watts per pound to have some power like you mentioned (I'm assuming you'll get the weight to . You'd want a mid-KV motor, I assume. It will probably be limited by your prop size. Highest-KV that can turn a 12".

I really don't know your budget for this plane... there's a wide range of brands from the cheap HK to the expensive Scorpion. I normally suggest Suppo motors from Altitude hobbies, because they are VERY good quality motors at great prices. Not amazing performers but very reliable and good quality control.

http://www.altitudehobbies.com/brush...v-suppo-3520-6

That motor with a 12x7 prop lands you right where you want to be, drawing 49A on a 4S. If your lightening is unsuccessful and you end up above 15-16 pounds, it'll still fly, but not with all the extra power. The problem is that any more power than that normally is a prop size up. So you should do your best to lighten it.

Hope that helps - and other people invited to post cause I might have screwed up too, it's late but still trying to help
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Old Aug 09, 2012, 11:23 AM
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United States, NC, Forsyth
Joined May 2010
117 Posts
I won't give a lot of details wthout actual testing, but I think you going
to need a higher Kv motor to get the desired power with your prop size
limit(12in), above 500kv ideal, most 60 size motors are around the 400
range, so you would have to use more cells for good power on a small
prop, these motors are designed to be used with larger props.

Two motors I have used and I believe to be suitable for Master Airscrew
12x8x3(3 blade), these will give around 150 watts/lb for good power for
a estimated 20 lbs plane IMO, better to have extra, you can always cut
throttle back!!. These are not cheap chinese, you have a costly plane,
it's worth the eaxtra expense IMO, for the quality. Both these will need
a HV 100 amp esc, Hobbywing and Exceed esc are good for the $$.

1st, HYPERION ZS4020 12 TURN 543KV, on 6 cells
http://www.allerc.com/hyperion-zs402...or-p-4599.html

2nd, Power 52 Brushless Outrunner Motor, 590Kv, on 5 cells
http://www.e-fliterc.com/Products/De...odID=EFLM4052A

Both motors are excellent quality IMO, my Hyperion wash destroyed last
year in a crash, I have the P52 in a 8 Lbs, 1/6 scale, ME163, good for aprox.
120 mph. The hyperion prices are insane, the E-flite is a good choice for $$.
BTW those plane weight numbers for the P52 on the link are conservative.

Have to go, LMK, Allen(ZRX212)
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Last edited by ZRX212; Aug 09, 2012 at 11:29 AM. Reason: more info
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Old Aug 10, 2012, 06:35 AM
hul
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Switzerland
Joined Mar 2003
2,094 Posts
this Excel spreadsheet can be used to calculate prop size and kv based on required power, voltage and pitch speed. Change the yellow cells, the others are outputs..
It pretty much confirms what Allen said.

Hans
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